A fluid accumulator is typically used in a hydraulic system for preventing the pressure of oil, the fluid, e.g., from rising excessively or suddenly falling due to temporary accumulation or discharging of oil contained in the hydraulic system. For example, the accumulator temporarily accumulates oil fed from a pump while a fluid-actuated device such as a hydraulic motor is turned off. The accumulator, when the device is actuated, discharges accumulated oil and feeds it to the device until more oil from the pump reaches the device.
The accumulator, as described above, usually comprises a cylindrical shell and a gas-filled rubber bladder incorporated therein, and the bladder is precharged with nitrogen gas under high pressure, for example, 2,900 lbf/in.sup.2 or 203.9 c kg/cm.sup.2. When the hydraulic system is pressurized to, for example, 5,000 lbs/in.sup.2 or 351.53 kg/cm.sup.2, the oil flows into the space between the shell and the bladder. The bladder is thereby deflated and internal pressure thereof rises. Oil continues to flow into the accumulator until the pressure in the bladder rises to 5,000 lbs/in.sup.2, at which time oil in volume equal to the reduction in volume of the bladder is accumulated. Contrarily, when oil in the accumulator is completely discharged therefrom, the pressure in the bladder decreases till 2,900 lbs/in.sup.2 and the bladder is restored to its initial shape.
Circumstances, such as flowing of oil into the accumulator when the fluid-actuated device is not operating, may be caused by various failures, e.g., clogging of the piping, leakage of gas from the bladder, and, as an extreme example, non-precharging of gas under predetermined pressure in the bladder. On the other hand, during the period that oil must be accumulated, accident interruption of the flow of oil into the accumulator may occur, e.g., due to oil leakage from the piping or due to trouble in any valve provided in the piping.
At present, however such accumulators are not available with a device for detecting inflation and deflation of the bladder and, therefore, troubles in fluid-actuated devices or piping in the hydraulic system can not be detected by reliance on an abnormal behavior of the bladder. For instance, trouble may not be found until the device works beyond a desired range or a quantity of oil has leaked out. The trouble, if not found at an early stage, develops further, and much time and labor may be required for repairing the system.